How to start modeling a square metal profile with holes?
You can start considering the object symmetries, 8 parts like that:



Thus, we'll try to make it and use Blender's mirror modifier.

As rounded parts are generally more complex to do, we start with them:

Add a circle, with a relative low resolution (say 16 vertices). This will be the center of the shape.

Then duplicate it in edit mode ShiftD and extrude and scale this last one ES to the wanted value.

Here you should have something like this:



As this is symmetrical, from the above picture, we can delete the unwanted part. So, only keep this:



From that, it is only aligning and extruding to be at the dimensions you want:



Now the mirror modifier part:

We need two mirrors, 


One to make a quarter of the shape: it uses an empty as pivot rotated 45 degrees.
One to replicate the previous one along X and Y axis.


Here is the setting:



So we now have:



Finally, we can add a solidify modifier to extrude the shape:



Last points:


Use shade smooth (in object mode W then 'shade smooth')
And use autosmooth for the normals (look at the image below)




A blend file with the several steps included: